1. Field of Invention
This invention relates to a method and apparatus for measuring torque and/or horsepower of a rotating object by creating and measuring avalanche Barkhausen reversals of the magnetic polarity in an element formed upon the rotating object.
2. Discussion of the Prior Art
To date, torque and/or horsepower measurement has not become a commonly used parameter for controlling rotating machinery due to the lack of a sufficiently small, lightweight, durable torquemeter. Torquemeters of the prior art use one of several approaches for measuring torsional stress within a rotating shaft. One approach generates torque-responsive signals from a strain gage cemented to the target shaft and passes these signals to an external receiver through electrical brushes in contact with slip rings on the shaft. The slip rings and brushes, however, degenerate from wear and the output signal of the strain gage is of low amplitude and exhibits erratic drift due to creep of the cement. More importantly, however, this approach requires that the strain gage and slip rings be attached to the rotating shaft and these additions often affect the original shaft balance in a manner which can reduce the life and performance of the machinery. Moreover, these devices generate signals which vary responsively only according to amplitude and they exhibit a poor signal at the higher rotational speeds due to the noise generated by the pick-up brushes. These shortcomings are of particular concern in connection with ultra-high speed rotating machinery such as rocket turbopumps, gas turbine engines and reciprocating engines, wherein the maintenance of shaft balance is critical and rotational speeds can be extreme. Moreover, these types of torque meters generate no indication of the rotational speed of the shaft, and thus they require an additional sensor such as a magnetic pickup device in order to determine rotary speed information necessary for the measurement of horsepower.
Another method for measuring the torsional stress in a rotating shaft employs a first magnetic coil to impart a bias magnetization within the shaft and a second coil in proximity to the first for detecting changes in the bias magnetization as the rotating shaft is placed uder torsional loading. A typical example of such an arrangement can be found in U.S. Pat. No. 3,861,206 to Kawafune et al. In another example, shown in U.S. Pat. No. 3,427,872 to Leep et al, a first coil and/or magneto-coil element induces magnetization into the test specimen and uses a second coil means for detecting Barkhausen noise as the biasing magnetization in the specimen is varied. However, the quality of the signal obtainable from these devices is highly dependent upon the rotational speed of the target shaft and are usually too faint and too slow in response time to give meaningful analysis of stress and/or torque in rapidly rotating shafts or the like. More importantly, Barkhausen noise evidences only a very weak signal. Consequently, the signal is sensitive to interference from other circuitry and its detection requires acutely sensitive and therefore costly sensing and amplification devices.
Yet another method of the prior art for measuring torque of a rotating subject employs a plurality of induction coils and a strain gage, wherein a first induction coil external to the rotating subject induces a current in a second coil secured to the rotating subject. The induced current is directed to a strain gage secured to the rotating subject by cement or other means. The strain gage then produces a signal which is responsive to the torsional strain carried by the rotating subject, which signal is conveyed to another external induction coil by means of yet another coil secured to the rotating subject. Because of the number of induction coils employed by this method and because of the losses associated therebetween, this method is extremely disadvantageous in terms of weight, power requirements and its effect on the original balance of the rotating subject. Furthermore, its output signals are usually so weak as to require extensive amplification, which requirement adds further to its complication and bulkiness as a system. Like the other methods, it also provides very unsatisfactory performance at the higher rotational speeds.